User:Yunzhong Hou
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n3
1(2) + 2(3) + 3(4) + ... + n(n + 1) = ?
an = n(n + 1)
Sn
12 + 22 + ... + n2 = (1(2) + 2(3) + ... + n(n + 1)) + (1 + 2 + ... + n)
1(2)(3)(4) + 2(3)(4)(5) + ... + n(n + 1)(n + 2)(n + 3) + (n + 1)(n + 2)(n + 3)(n + 4)